Although various industries are extant which employ processes for forming a thin layer or film on a solid substrate, one significant industry in which such processes are widely employed is the production of semiconductor devices. In such production processes, substrates such as planar silicon or gallium arsenide wafers or other suitable such materials are exposed to gases which react to deposit the desired materials on the surface of the substrate or wafer. In typical processes of this nature, the deposited materials thus form epitaxial films which replicate the crystal lattice structure of the underlying substrate.
These coated wafers are then subjected to well known further processes to form devices such as integrated circuits. The layers deposited on the wafer form the active elements of microscopic transistors and other semiconductor devices included in the integrated circuits. The thickness, composition and quality of the deposited layers determine the characteristics of the resulting semiconductor devices. Accordingly, the deposition process must be capable of depositing films of uniform composition and thickness on the front face of each substrate. The requirements for uniformity have become progressively more stringent with the use of larger wafers and with the continuing reduction in the size of the semiconductor devices fabricated from the coated wafers.
Several processes are currently utilized in the semiconductor industry to deposit these very thin epitaxial layers on wafer substrates. Among these, the more significant and widely employed processes include molecular beam epitaxy, chemical beam epitaxy and metallorganic chemical vapor deposition. All of these techniques have inherent drawbacks which either diminish the quality of the resultant semiconductor devices or render the deposition process lengthy, difficult and costly.
In a typical molecular beam epitaxy (MBE) system, two or more source materials are separately heated in effusion cells to thus generate individual beams consisting of molecules (or atoms) of these materials. The individual beams of molecules then travel under molecular flow conditions toward the surface of a heated substrate where they react to deposit a layer of a desired composition on the substrate surface. As used herein, the term "molecular flow" refers to material flow in which individual molecules of the material can move freely without colliding with one another. Molecular flow through a vessel exists where the mean free path of a molecule (i.e. the distance traveled without collision) is greater than the smallest characteristic dimension of the vessel. Although widely used, the molecular beam epitaxy technique has many inherent deficiencies which plague the production process and limit the coating quality obtainable therewith. Firstly, the proportion of source materials to be deposited in a particular layer can only be changed by adjusting the temperatures of the individual effusion cells. Not only does this provide a woefully inadequate level of control, but it slows the production process since it requires extended lengths of time for the effusion cells to reach the required temperature. During this temperature adjustment period, the flow of the source material beams must be interrupted by positioning shutters between the effusion cell sources of the beams and the substrate target. Since these shutters do not make a physical seal they leak, and therefore are ineffective in completely preventing any additional deposition. A further deficiency of the molecular beam epitaxy process is that it inherently causes the formation of defects in the deposited layer. Such defects are caused, for example, by microscopic droplets which form when the vapor pressure immediately above the molten source material causes condensation or "spitting" or when molecules of material from one source beam collide with molecules of material from another source beam, the microscopic droplets then adhering to the surface of the substrate to form what are typically referred to as oval defects. Finally, the molecular beam epitaxy deposition process must be conducted at very high vacuum levels on the order of 10.sup.-11 Torr, and therefore requires the use of highly specialized and very expensive equipment.
In the alternate coating process of metallorganic chemical vapor deposition (MOCVD), gaseous hydride sources and metallorganic vapor sources flow with a carrier gas through a plurality of apertures in a diffuser plate wherein the gas flow is divided into a plurality of individual streamlines or flow lines directed at the substrate. A heated susceptor in close proximity to the substrate causes the gases to decompose into elemental materials as they hit and become deposited on the substrate. The composition of the deposited layer is determined by adjusting the proportional flow rate of the various source gases, and is therefore able to be controlled more accurately than with molecular beam epitaxy. The interface abruptness between adjacent layers is developed by controlling the source material flow between a run condition and a vent condition without leakage. Furthermore, the metallorganic chemical vapor deposition process may be conducted at relatively low vacuum levels of about 60 Torr, therefore overcoming the stringent equipment requirements associated with molecular beam epitaxy. This process, however, effects an entirely different set of problems, the most significant of which are associated with the source materials themselves since the metallorganic gases required for this process are not only expensive to produce, but are also dangerous to use and dispose of because of their explosiveness and toxicity. Moreover, these gases exhibit viscous flow characteristics as they travel from the diffuser plate for deposition on the substrate. Such flow cannot be effectively controlled and consequently results in the formation of surfaces on the substrates which are non-uniform with respect to both thickness and composition. In addition, the decomposition of the source gases at or near the surface of the substrate often results in carbon contamination of the deposited layer.
The chemical beam epitaxy (CBE) deposition process combines the attributes of both molecular beam epitaxy and metallorganic chemical vapor deposition. In accordance with this process, gaseous sources are fed to cracking cells from which they flow towards a heated substrate. While some gases decompose in the cracking cells, others decompose at the surface of the heated substrate. Since the chemical beam epitaxy process does not require the use of toxic metallorganic source gases, it overcomes those problems inherent in metallorganic chemical vapor deposition which are attributable to the use of these materials. However, chemical beam epitaxy must be performed at higher vacuum levels than MOCVD which are on the order of about 10.sup.-5 Torr. Moreover, the growth rate of the deposited layer is dependent on the temperature of the substrate surface, and is therefore difficult to control, while the deposited layer is again subject to carbon contamination due to the decomposition of the gases near the substrate surface.
In view of the deficiencies of the aforementioned deposition processes, it has been extremely difficult heretofore to consistently produce coated wafers having substantially no defects. Accordingly, there is a need for an improved process and apparatus which is capable of depositing defect-free thin layers on a substrate in an economical manner. Not only should such process be easy to perform and control, but it should eliminate the need to use toxic and/or expensive source materials, and should use the required materials efficiently. Furthermore, the process should not require the use of highly specialized equipment which is troublesome to operate. In other words, there is a need for both a commercially viable process and an apparatus for performing that process which can provide for the deposition of more uniform layers on substrates with substantially no defects.